1. Field of the Invention
The present invention relates generally to routing protocols. More particularly, the present invention relates to a soft state routing protocol for mobile wireless sensors in a wireless sensor network.
2. Description of Related Art
A wireless sensor network (WSN) is a self-organizing wireless network that consists of distributed devices with sensors commonly referred to as “nodes”. The sensors are of application specific. They are used for monitoring environmental conditions, such as sound, temperature, vibration, pressure, motion or pollutants at different locations. The wireless sensor network was originally motivated by military applications. It is now widely used in many industrial, agricultural, medical and other civilian applications.
In addition to the sensors, each device in a WSN is equipped with elements such as a radio transceiver, microcontroller, flash memory, antenna, energy source and so on. A WSN device is typically low data rate, low cost and low power devices. The low cost of a WSN is a constraint on system resources such as energy, memory, processor and bandwidth.
IEEE 802.15.4 is a standard which specifies the physical layer and media access control for Wireless Sensor Network. In this standard, there are two types of multi-access mechanisms: beacon and non-beacon. In a beacon enabled network, nodes transmit in specific time slots. The sink node, also referred to as the coordinator periodically sends a beacon frame. All nodes in the WSN are expected to synchronize to this frame. Each node is assigned a specific slot to transmit and receive its data. There is also a common slot during which all nodes compete to use the channel. In a non-beacon enabled network, all nodes in a network can transmit at any time when the channel is available.
There are usually three types of WSN devices, i.e. the coordinator (or sink), router and end device. The first two devices have routing functionality. Both devices have ability to find proper routes to forward packets to desired destinations. An end device has no routing ability, it only communicates with a specific router that is called the parent of the end device.
If the routing condition changes so that the original path becomes unavailable, the routers are able to find a new path to deliver the packets to the destination. Currently, the WSN devices are usually installed in fixed locations. However, if the devices were installed inside a moving target, the routing among the wireless nodes becomes a challenge.
Currently, there are a few routing solutions for partially mobile WSN:
Mobile base station (MBS) based solutions: A MBS is a mobile sink that changes its position within a WSN. The packets are sent to the MBS via relay node.
Mobile data collector (MDC) based solutions: A MDC is a mobile sink that visits sensors. The packets are buffered at source nodes until the MDC visits the sensors. The data is sent over to the MDC without relay nodes.
Rendezvous based solutions: The data is sent to rendezvous nodes that are closer to the mobile sink. The packets are buffered at the rendezvous nodes until the sink receives them.
However, these solutions are restricted in that all the nodes must be fixed except the sink node. Therefore, the mobility is substantially limited.
Further some basic soft state refreshment methods exist such as:
A specific timer that is associated with each individually sent refreshing message. This approach is inefficient and consumes copious resources for refreshing processes.
In another soft state method, the refreshing messages are flooded to the network altogether once the timer fires. This approach can overwhelm local devices, creating broadcast storms over the network, hence a packet can be lost. On the other hand, the refreshing process is in sleeping mode and does nothing before the timer triggers. This brute force approach eliminates the possibility to customize the refreshment.
Another soft state method involves bundling a subset of the refreshing messages into a single packet which is sent to the wire when the timer fires. This is a compromise to the previous two methods. Still, this approach cannot provide a resolution to take care of specific requirements.
The third approach is an improvement to the previous two ways, however, in general the current approaches perform refreshing action with very limited intelligence.
Therefore, what is needed is a routing protocol that may efficiently allow for all nodes to be mobile. Moreover, a more efficient soft state solution may be desirable to enhance routing functionality in a mobile WSN.